1. Field of the Invention
The present invention relates to the diffusion of impurities into semiconductor bodies, more particularly, to an improved solid-solid diffusion process.
2. Description of the Prior Art
Vapor diffusion processes for selectively introducing impurities into semiconductor substrates are well known. In these processes an impurity-containing compound is brought into contact in the vapor phase with a semiconductor body in a low temperature "pre-deposition" diffusion followed by a subsequent "drive-in" diffusion, the surface impurities being driven into the semiconductor body to the desired depth dependent upon the diffusion time and temperature.
A substantial disadvantage encountered with the above type or processing is that different process conditions are required for the "predeposition" and the "drive-in" diffusion.
Recently, diffusion processes have been developed which require only a "drive-in" diffusion based upon a solid diffusion source which is deposited directly on the exposed surface area of a semiconductor body into which the diffusion is to be performed.
The semiconductor body, optional diffusion mask if desired, and diffusion source are heated to an elevated temperature to drive impurities from the solid diffusion source into the semiconductor body.
Such solid sources include "spin-on" or "paint-on" glasses, which can be spun onto a semiconductor body by centrifugal force as a thin coating. Spin-on doping compositions are disclosed, for example, in U.S. Pat. Nos. 3,532,563 and 3,615,943 to Genser.
Such "spin-on" or "paint-on" glasses have generally proven satisfactory for the formation of shallow PN junctions or diffused regions of relatively low impurity concentration, but have not proven satisfactory for forming a deep-diffused PN junctions or heavily doped diffused regions. For instance, such glasses generally cannot be used to form a deep diffused junction on the order of 3 to 4 mils. Further, such glass sources become relatively rapidly depleted of impurities when subjected to relatively long temperature cycles as are required for deep or high concentration diffusions with such compositions.
A recent development in the art has been the use of doping or dopant papers to perform solid-solid diffusions into a semiconductor body. Dopant papers have generally been used in a manner similar to "spin-on" or "paint-on" glasses by contacting them with the semiconductor body into which the diffusion is to occur and elevating the temperature of the total assembly, whereby impurities diffuse from the dopant paper into the semiconductor substrate.
Such dopant papers are commercially available from Filmtronics Inc., Butler, Pennsylvania, and generally comprise an organic binder which contains an organic solvent which includes a desired P- or N-type impurity. Such materials are most commonly available under the general specification of "boron dopant paper" or "phosphorus dopant paper", and generally contain high amounts of dopant, e.g., on the order of 40 weight percent boron and 60 weight percent phosphorus for "boron" and "phosphorus" dopant paper, respectively. This general type of doping paper is also disclosed in U.S. Pat. No. 3,630,793 (Christensen et al.).
The use of such dopant papers has, however, been subject to various faults. For example, with phosphorus dopant papers irregular surface attack on the semiconductor body where the dopant paper touches the semiconductor occurs during diffusion, resulting in a non-uniform or wavy junction, highly objectionable from the device manufacturer's point of view.
Even if such dopant papers are separated from the semiconductor body by a film of silicon dioxide a non-uniform or wavy junction still results, as illustrated in FIG. 4 later discussed.